Confused and really need advice :/

This is kind of an OCD thing, but also a rant that I really don’t know the answer to. I reconnected to an old friend, because I really needed some advice after my break up. I had ROCD regarding if I secretly liked him, and proved myself wrong but I still felt a type of way once I saw him. He showed me the apartment I missed out on, because I was never able to see him during the relationship. We talked and I laughed so hard like I hadn’t in months. My face turned red when he paid close attention to me. He talked about his upcoming career in medicine- my ex had no clue what he was going to do, and was spiraling into 🍃 problems which I absolutely despised. I realize that I was having so much fun with this friend, I genuinely wanted to ask him out. After all the advice he had given I felt so much more free, so much more understood, and like I could laugh about this problem that was weighing me down. He knew how I felt regarding my ex, but said yes, and tried to plan something with me. The problem is when I went back to his apartment the next time, I felt weird. We talked about the one time that we got together the summer after high school (we have a TINY bit of backstory) and how funny and unserious it was. But I was hit with a pang of absolute anxiety and sadness. I liked these conversations with my ex, and I was having them with a whole different person. I didn’t want to be intimate with anyone but my ex. And I was sitting there having a great time with this friend, feeling awful as I spiraled into what could’ve been. I stayed talking with him until very late at night because we were still having a great time, but when he suggested I stayed I declined quickly, even though I had my work stuff sitting in my car prepared for that option. Now I just feel empty.. and confused. My ex tried reaching out during this whole thing and I declined thinking that I found something better and needed to not give it more attention.. I was wrong. I really don’t know what to do now.